Clearly we are all prone to errors, as humans. That's why the use of a well designed checklist is so important - precisely to catch items missed. NASA refer to them as 'killer' items : a little melodramatic perhaps but meant to emphasise that what's in a checklist should be important and other stuff should be left to SOP.
Regarding the floats, what speed have they been certified to for deployment? There have been floatation systems around for a while now, that are armed in th e cruise. Bristow introduced them for the 332 about 10 years ago. The EC-225 floats are armed in the cruise, as will the EC-175 ones. I would have thought it would be essential that the manufacturer demonstrates deployment at whatever arming speed they choose to nominate, and ceratinly AC29-2C implies this.